In the prior art, it is well known that syngas (i.e. CO and H.sub.2) can be produced from methane (or natural gas) by following different catalytic processes.
Steam reforming of methane: It is a highly endothermic process and involves following reactions:
Main reaction PA0 Side reaction PA0 Main reaction PA0 Side reaction: Reverse water gas shift reaction PA0 Main reaction PA0 Side reaction
CH.sub.4 +H.sub.2 O.dbd.CO+3H.sub.2 -54.2 Kcal per mole of CH.sub.4 at 800.degree.-900.degree. C. PA1 CO+H.sub.2 O.dbd.CO.sub.2 +H.sub.2 +8.0 kcal per mole of CO at 800.degree. C.-900.degree. C. PA1 CH+CO.dbd.2CO+2H.sub.2 -62.2 kcal per mole of CH.sub.4 at 800.degree.-900.degree. C. PA1 CO.sub.2 +H.sub.2.dbd.CO+H.sub.2 O-8.0 kcal per mole of CO.sub.2 at 800.degree.-900.degree. C. PA1 CH.sub.4 +0.5 O.sub.2.fwdarw.CO+2H.sub.2 +5.2 kcal per mole of CH.sub.4 at 500.degree.-800.degree. C. PA1 CH.sub.4 +2O.sub.2.fwdarw.CO.sub.2 +2H.sub.2 O+191.5 kcal per mole of CH.sub.4 at 500.degree.-800.degree. C. PA1 CO+H.sub.2 O.dbd.CO.sub.2 +H.sub.2 PA1 1) The catalysts are prepared by depositing nickel oxide, with or without other catalytically active metal oxides, directly on refractory supports viz. alumina, silica and zirconia, and calcining at high temperatures. Hence during the calcination, the deposited metal oxides undergo solid-solid reactions with Al.sub.2 O.sub.3, SiO.sub.2 and ZrO.sub.2 resulting in the formation of catalytically inactive binary metal oxide phases such as nickel aluminate, nickel silicate and zirconium nickelate, respectively. These binary metal oxide phases are very difficult to reduce and are also catalytically inactive. Even after the reduction of the catalyst to metallic Ni, these inactive binary metal oxide phases reappear during its long operation in the process and also during catalyst regeneration by burning of coke on the catalyst. Therefore, the catalysts have poor stability, low activity and selectivity and also show low efficiency (i.e. productivity for CO and H.sub.2) in the methane or light hydrocarbons-to-syngas conversion processes. PA1 2) The catalysts operate at high temperatures, at least at 1700.degree. F. (i.e. 927.degree.), in the oxidative conversion of methane or natural gas to syngas. PA1 3) In most of the cases, the catalysts are reduced by hydrogen before using them in the methane or light hydrocarbons-to-syngas conversion processes. PA1 4) The catalysts when used in unreduced form show activity in the oxidative conversion of methane to syngas only at high temperatures; the reaction is initiated only above 750.degree. C. PA1 1. to provide process for the catalytic conversion of methane or natural gas to syngas (i.e. a mixture of CO and H.sub.2) through using the improved supported catalyst disclosed in our copending patent application Ser. No. 08/359,035, now abandoned, which has a number of above mentioned advantages over the earlier supported nickel containing catalysts described in the prior art for the oxidative conversion of methane or light hydrocarbons to syngas. PA1 2. to provide a process for the catalytic conversion of methane or natural gas to syngas in a most energy efficient manner and also in a very safe manner requiring little or no external energy for the conversion through coupling of the exothermic oxidative conversion with oxygen of methane or natural gas to syngas with the endothermic steam and CO.sub.2 reforming of methane or natural gas to syngas by carrying out these exothermic and endothermic reactions simultaneously over the improved catalyst in a simple fixed bed reactor operated adiabatically or non-adiabatically. PA1 3. to provide a process for the catalytic conversion of methane or natural gas to syngas, which can be operated continuously at a temperature below 900.degree. C. (i.e. 1652.degree. F.) for a long period of time without deposition of carbon or coke on the catalyst as well as without losing the activity of the catalyst, without lowering its mechanical strength and/or without disintegrating it into powder during application. PA1 a) mixing oxygen with carbon dioxide and methane or natural gas at ambient temperature, PA1 b) preheating the steam and mixture of oxygen, carbon dioxide and methane or natural gas to a temperature between about 600.degree. C. and about 900.degree. C., PA1 c) admixing said preheated steam with said preheated mixture of oxygen, carbon dioxide and methane or natural gas, PA1 d) passing continuously the resulting admixture feed over the said improved supported catalyst in a fixed bed reactor operated adiabatically or non-adiabatically, maintaining the mole ratio of organic carbon (i.e. carbon in hydrocarbon) to oxygen, steam and carbon dioxide in said admixture feed between about 1.8 and about 2.8, between about 1.1 and about 25 and between about 2.0 and about 50, respectively, gas hourly space velocity of said admixture feed between about 2000 cm.sup.3.g..sup.-1 h.sup.-1 and about 200,000 cm.sup.3.g.sup.-1.h.sup.-1, a reaction temperature between about 650.degree. C. and about 925.degree. C., and a pressure between about 1 atm and about 50 atm such that an effluent is produced containing carbon monoxide and hydrogen in a mole ratio of hydrogen to carbon monoxide between about 1.5 and about 3.0 and containing less than about 4 mole percent unreacted methane or natural gas with about 100% selectivity for both CO and H.sub.2 in the conversion of methane or natural gas. PA1 (ii) Oxidative conversion of methane or natural gas with oxygen to carbon dioxide and water, which is a highly exothermic reaction, involving evolution of about 191 kcal heat per mole of organic carbon converted. PA1 (iii) Steam reforming of methane or natural gas to carbon monoxide and hydrogen giving H.sub.2 /CO ratio of about 3.0, which is highly endothermic reaction, involving an absorption of about 54 kcal heat per mole of organic carbon converted. PA1 (iv) Carbon dioxide reforming of methane or natural gas to carbon monoxide and hydrogen giving H.sub.2 /CO ratio of about 1.0, which is also a highly endothermic reaction, involving an absorption of about 62 Kcal heat per mole of organic carbon converted. PA1 (v) Reaction of carbon dioxide with hydrogen to carbon monoxide and water, a reverse shift reaction, which is a mildly endothermic reaction involving an absorption of about 9 Kcal heat per mole of carbon dioxide converted. PA1 (vi) Reaction of steam with carbon monoxide to carbon dioxide and hydrogen, a shift reaction, which is a mildly exothermic reaction involving an evaluation of about 9 Kcal heat per mole of carbon monoxide converted.
CO.sub.2 reforming of methane: It is also a highly endothermic process and involves the following reactions:
Partial oxidation (i.e. Oxidative conversion) of methane: It is an exothermic process and involves following reactions:
Use of nickel containing catalysts, particularly nickel (with or without other elements) supported on alumina or other refractory materials, in the above catalytic processes for conversion of methane (or natural gas) to syngas is also well known in the prior art. Kirk and Othmer, Encyclopedia of Chemical Technology, 3rd Ed., 1990, vol. 12, p. 951; Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, pp. 186 and 202; U.S. Pat. No. 2,942,958 (1960); U.S. Pat. No. 4,877,550 (1989); U.S. Pat. No. 4,888,131 (1989); EP 0 084 273 A2 (1983); EP 0 303 438 A2 (1989); and Dissanayske et al., Journal of Catalysis, vol. 132, p. 117 (1991).
The catalytic steam reforming of methane or natural gas to syngas is a well established technology practiced for commercial production of hydrogen, carbon monoxide and syngas (i.e., a mixture of hydrogen and carbon monoxide). In this process, hydrocarbon feed is converted to a mixture of H.sub.2, CO and CO.sub.2 by reacting hydrocarbons with steam over a supported nickel catalyst such as NiO supported on alumina at elevated temperature (850.degree. C.-1000.degree. C.) and pressure (10-40 atm) and at steam to carbon mole ratio of 2-5 and gas hourly space velocity of about 5000-8000 per hour.
This process is highly endothermic and hence it is carried out in a number of parallel tubes packed with a catalyst and externally heated by flue gas to a temperature of 980.degree.-1040.degree. C. (Kirk and Othmer, Encyclopedia of chemical Technology, 3rd, Ed., 1990, vol. 12, p. 951, Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, p. 186).
The main drawbacks of this process are as follows: It is highly endothermic and operated at high temperature. Hence, it is highly energy intensive. Further, the water gas reaction occurring in the process leads to formation of CO.sub.2 and H.sub.2 from CO and water, thus increasing H.sub.2 /CO ratio. Since lower H.sub.2 /CO ratio than that obtained by the steam reforming is required for certain applications of syngas, secondary reformer using CO.sub.2 or O.sub.2 oxidants are frequently required to reduce the hydrogen content of syngas produced by the steam reforming. Also, there is a carbon deposition on the catalyst during the steam reforming.
Autothermal catalytic reforming of methane or natural gas with air or oxygen to hydrogen, carbon monoxide and carbon dioxide is also an established technology. In this process, a feed gas mixture containing hydrocarbon, steam and oxygen (or air) is passed through a burner and then the combustion gases are passed over a catalyst, nickel supported on alumina, in a fixed bed reactor at 850.degree.-1000.degree. C. and 20-40 atm. (Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, p. 202). This process has following drawbacks: There are large temperature and space velocity variations during start-up and shut-down which leads to abrasion and catalyst disintegration, requiring frequent refilling and removal of the catalyst. This process operates at high temperature and pressure and there is a formation of carbon or coke in the reactor.
U.S. Pat. No. 2,942,958 (1960), disclosed a process for the conversion of a normally gaseous hydrocarbon to carbon monoxide and hydrogen, which comprises preheating of normally gaseous hydrocarbon and steam to a temperature between 538.degree. and 760.degree. C., admixing the preheated gaseous hydrocarbon and steam with oxygen and contacting the resulting admixture with a catalyst which comprises a nickel oxide supported on a refractory material such as zirconia or other refractory metal oxide support in a fixed bed reactor at a reaction temperature between 1800.degree. F. (i.e. 982.degree. C.) and 2200.degree. F. (i.e. 1204.degree. C.) and pressure between 150 and 350 psig, maintaining the mole ratio of oxygen and steam to methane in the feed between 0.3 and 0.7 and between 1 and 2, respectively. The main drawback of this process is that the process operates at very high temperature (i.e., above 982.degree. C.). Another drawback of this process is that it is hazardous to mix oxygen with the preheated mixture of gaseous hydrocarbon and steam.
In U.S. Pat. No. 4,877,550 (1989) and U.S. Pat. No. 4,888,131 (1989), Goetsch et. al. have disclosed a fluid bed process for the production of syngas which comprises reacting a light hydrocarbon feed with steam and oxygen at least about 1750.degree. F. (i.e. 954.4.degree. C.) and about 1700.degree. F. (i.e. 926.7.degree. C.), respectively, in the presence of a supported nickel catalyst, nickel supported on a alumina having particle size in the range of 30-150 microns, in a fluid bed (i.e., fluidized bed) reactor. The main drawbacks of this process are as follows: The process operates at very high temperature. Further, this process involves a use of a fluid bed reactor which is extremely difficult to scale-up, design and operate for the process, particularly at a very high reaction temperature employed in the process. Also, the product gas of the process contains entrained catalyst (i.e. catalyst particles are carried away by the product gases during the process) which is to be removed from the product gases, the entrained catalsyt catalyses side reactions such as water gas reaction leading to the conversion of CO to CO.sub.2, thus reducing the selectivity for carbon monoxide during the cooling of the product gases, because the water gas reaction is favoured at lower temperatures.
European Patent EPO 303,438, describes a process for the production of syngas from hydrocarbonaceous feedstock such as natural gas by a catalytic partial oxidation process wherein natural gas is mixed with steam in a steam to carbon molar ratio in the range from 0.3 to 2.0, the natural gas and steam mixture is heated to a temperature in the range from 340.degree. to 650.degree. C., the resulting preheated mixture of natural gas and steam is admixed with oxygen or oxygen containing gas preheated to a temperature from 65.degree. C. 650.degree. C., the mixture of natural gas, steam and oxygen is then passed over a catalyst comprising Pt, Rh, Ir, Os, Ru, Pd, Ni, Cr, Co, Ce, La, and a mixture thereof supported on a monolithic structure containing alumina, zirconia, mullite, aluminium titanate, cordlerite etc., with a space velocity from 20,000 h.sup.-1 to 500,000 h.sup.-1 at a reaction temperature in the range from 760.degree. C. to 1090.degree. C. to produce a synthesis gas consisting essentially of hydrogen, carbon oxides and steam. The main drawbacks of this process are as follows: it is hazardous to mix oxygen with the preheated mixture of natural gas and steam. Because of the high steam to carbon mole ratio in the feed, the H.sub.2 to CO mole ratio in the product is always above 2.0.
European Patent EPO 084 273 (1983) discloses production of carbon monoxide and hydrogen with H.sub.2 /CO mole ratio of about 1.0 from C.sub.2 -C.sub.4 olefins and C.sub.1 -C.sub.4 paraffins by their reaction with carbon dioxide using a catalyst containing iron, cobalt or nickel supported on silica at a reaction temperature in the range from 350.degree. to 850.degree. C. and pressure of 1-2 atm. The main drawbacks of this process are as follows: this process involves a highly endothermic reaction of CO.sub.2 with hydrocarbons and hence it is a highly energy intensive process. In this process, the H.sub.2 /CO mole ratio in the product is restricted to about 1.0. Further, it is well-known in the prior art that, in the CO.sub.2 reforming of hydrocarbons to syngas, the coke formation on a catalyst is very fast and hence catalyst deactivation due to coking is very fast.
Since the steam reforming (Reaction-1) or CO.sub.2 reforming (Reaction-3) of methane or natural gas to syngas is highly endothermic the processes based on the steam and/or CO.sub.2 reforming of methane or natural gas to syngas are highly energy intensive. These processes also suffer from the drawback of coke formation and consequently fast catalyst deactivation due to coking in the process. Whereas in case of the oxidative conversion with oxygen of methane or natural gas to syngas, the exothermic and therefore no external energy is required for the conversion of methane or natural gas to syngas and so there is little or no coke formation in the process. However, this process also suffers from a number of severe limitations such as (i) problems of the removal of high exothermic heat of reaction from the reaction zone because a large amount of heat is produced in a very small reaction zone due to high conversion coupled with very high space velocity, (ii) requirement of a complicated reactor e.g. fluidized bed reactor with heat exchangers and consequently very high capital cost and process operation cost; the process in fluidized bed reactor is very difficult to design and scale-up; (iii) further, there is a high possibility of run-away conditions and hence the process is highly hazardous or its operation is not safe.
Apart from the limitations of the earlier processes described above, there are following main disadvantages of the supported nickel catalysts described in the prior art for their use in the oxidative conversion of methane or light hydrocarbons to syngas conversion processes described in the prior art.
In order to overcome the above limitations or drawbacks of supported nickel containing catalysts described in the prior art, an improved catalyst has been developed which has been made the subject matter of our co-pending application Ser. No. 08/359,035, now abandoned.
In the said copending application a process for the preparation of an improved supported catalyst containing oxides of nickel and cobalt, with or without noble metals, deposited on a precoated support, useful for the oxidative conversion of methane, natural gas and biogas to syngas by different processes involving partial oxidation with oxygen or oxidative steam and/or CO.sub.2 reforming with oxygen of methane or light hydrocarbons to syngas; the improved supported catalyst is represented by the formula: EQU A.sub.a Co.sub.b NiO.sub.c (x)/MO.sub.d (y)/S,
wherein, A is noble metal elements selected from Ru, Rh, Pd, Pt, Ir, Os, or a mixture thereof, Co is cobalt, Ni is nickel, O is Oxygen, M is alkaline earth element selected from Be, Mg, Ca or a mixture thereof, a is A/Ni mole ratio in the range of 0 to about 0.1, b is Co/Ni mole ratio in the range of about 0.01 to about 2.0, c is number of oxygen atoms needed to fulfill the valence requirement of A.sub.a Co.sub.b Ni, d is the number of oxygen atoms required to fulfill the valence requirement of M, S is a catalyst support selected from sintered low surface area porous refractory inert solids comprising alumina, silica, silica-alumina, silicon carbide, zirconia, hafnia or a mixture thereof, y is weight percent loading of MO.sub.d precoated on the support in the range of about 0.3 wt % to about 30 wt %, and x is wt % loading of A.sub.a Co.sub.b NiO.sub.c deposited on the precoated support in the range of about 0.3 wt. % to about 30 wt. %, and prepared by precoating support with MO.sub.d and then depositing A.sub.a Co.sub.b NiO.sub.c on the precoated support.
Main advantages of the improved supported catalyst over the earlier supported catalysts containing nickel, useful for the conversion of methane or light hydrocarbons to syngas are as follows: (i) The improved supported catalyst is prepared by depositing oxides of nickel and cobalt, with or without noble metals, on a sintered low surface area porous inert support, surface of which is precoated with an oxide of Be, Mg, Ca or a mixture thereof so that a protective layer of the alkaline earth oxide is formed between the support and the oxides of nickel and cobalt, with or without noble metal, and hence direct chemical interactions between the oxides of the Group VIII transition metals and the reactive components of support, which leads to the formation of catalytically inactive binary oxide phases, which are very difficult to reduce, by solid-solid reactions on the support surface are avoided and thereby the catalyst shows much higher activity, selectivity and productivity, operates at lower temperatures and higher space velocities, does not deactivate due to the formation of catalytically inactive binary metal oxide phases in its operation in a long run, and also the catalyst can be used in its unreduced form and the reaction on the unreduced catalyst is initiated or started at much lower temperatures, in the oxidative methane or light hydrocarbons-to-syngas conversion processes. (ii) In the improved supported catalyst, nickel and cobalt are present together producing a synergetic effect thereby increasing resistance to coke deposition on the catalyst and also enhancing its catalytic activity and selectivity in the oxidative methane or light hydrocarbons-to-syngas conversion processes. The addition of cobalt to the catalyst also reduces the reaction start temperature for the catalyst in its unreduced form. (iii) The reaction start temperature of the improved supported catalyst in its unreduced form is further decreased by the presence of noble metal in the catalyst at low concentrations. (iv) Because of the use of a sintered low surface area porous inert support comprising a refractory material, the improved supported catalyst is thermally very stable and also has high mechanical strength and attrition resistance.
The present energy crisis and/or high energy cost and also the environmental pollution problems have created a great need for developing a catalytic process for the conversion of methane or natural gas to syngas, which requires little or no external energy, operates in most energy efficient manner and also has absolutely no hazards (i.e. very safe operation). Hence, there is a need to develop a process for the oxidative conversion of methane or natural gas to syngas in an energy efficient manner using an improved supported catalyst so that a most of the drawbacks or limitations of the earlier processes could be overcome. This invention was therefore, made with the following objects: